1. Field of the Invention
The present invention relates to a focal-plane shutter device of a camera.
2. Related Background Art
This kind of shutter device has the following problem: when a blade of aperture closing blades for forming an exposure slit is braked just before it covers an aperture and stops running, the center portion of the slit forming blade is bent and collides with a base plate. An example of a conventional device to solve this problem is disclosed in, for example, U.S. Pat. No. 4,493,544. The conventional device will be described in brief with reference to FIGS. 4A to 4C.
In order to make the description plain, only a first base plate 1, a second base plate 2, a blade 3 of aperture closing blades for forming an exposure slit and a blade 4 of aperture opening blades for forming the exposure slit are shown in FIGS. 4A to 4C. FIG. 4A is a perspective view showing a state in which the aperture closing blades have completed running and have covered an exposure aperture after the aperture opening blades have run, opened the exposure aperture and exposed a film plane. FIG. 4B is a front view of the state and FIG. 4C is a sectional view of a portion indicated by arrows C--C in FIG. 4B.
The first base plate 1 has a rectangular aperture bounded by edges 1a, 1b1c and 1d, and the second base plate 2 has a rectangular opening bounded by edges 2a, 2b, 2c and 2d. A rectangular aperture A, which has the edges 1a, 2a, 1c (2c) and 1d (2d) as shown in FIG. 4B, is formed by overlapping the first base plate 1 and the second base plate 2. The edges 1c and 2c are exactly aligned with the same plane and the edges 1d and 2d are exactly aligned with the same plane. A slit forming edge 3a of the aperture closing blades is positioned a predetermined width W.sub.1 lower than the edge 2a in the running direction of the aperture closing blades (downward in FIG. 4C) and constitutes an overlapping portion having the predetermined width W.sub.1. The edge 1b is positioned a predetermined width W.sub.2 lower than the slit forming edge 3a in the running direction of the aperture closing blades and forms an opening having the predetermined width W.sub.2. Therefore, a concave portion R.sub.1 having a width of W.sub.1 +W.sub.2 and a depth corresponding to the thickness t.sub.1 of the first base plate 1 is formed between the edges 2a and 1b. Similarly, a concave portion R.sub.2 having a depth corresponding to the thickness t.sub.3 of the second base plate 2 is formed between the edges 2b and 1a.
The slit forming blade 3 extends to cross the aperture A and is slidable upward and downward, and both edges thereof overlap the first base plate 1. The slit forming blade 3 moves together with an unillustrated divided cover blade downward in FIG. 4C, closes and shades the aperture A, moves upward, and opens the aperture A to make an exposure operation.
The slit forming blade 4 almost similarly extends to cross the aperture A and is slidable upward and downward, and both edges thereof overlap the second base plate 2. The slit forming blade 4 moves upward in FIG. 4C, closes and shades the aperture A, moves downward and opens the aperture A to make an exposure operation.
Before a shutter release operation is performed, the aperture opening blades including the blade 4 cover the aperture A and the aperture closing blades including the blade 3 are put away upward in the figure to open the aperture A.
When the shutter release operation is performed, the aperture opening blades, which have shaded the aperture A, move downward and start an exposure operation. The blade 4 stops at the position shown in FIG. 4C and is put away. When a predetermined time has passed after the aperture opening blades start to move, the aperture closing blades move downward to shade the aperture A. Then, the blade 3 stops at the position shown in FIG. 4C and the exposure operation is completed. Even if the running blade 3 of the aperture closing blades is braked just before it completes its ruining operation and the center portion of the blade 3 is bent by kinetic energy in the direction of the optical axis, since the edge 2a of the aperture A is spaced by the distance corresponding to the depth t.sub.1 of the concave portion R.sub.1 from the blade 3, the blade 3 does not collide with the edge 2a. Furthermore, since there is a space having the predetermined width W.sub.2 between the slit forming edge 3a of the blade 3 and the edge 1b of the first base plate 1, the edge 3a does not collide with the edge 1b.
The above-mentioned art is advantageous in a conventional shutter device in which a blade for forming a slit is made of a rigid material such as iron and the running speed of the blade is comparatively low.
However, if the shutter blade is made of a light material, such as aluminum, titanium, or a compound material thereof, which has recently been put into practical use, so as to obtain a high shutter speed, since the rigidity of the slit forming blade is lower than that of the blade made of an iron or the like, the degree to which the center portion of the blade is bent by braking the blade running at high speed is increased, and the space corresponding to the thickness of the base plate in the conventional device is insufficient. As a result, the edge of the blade collides with the edge of the opening of the base plate and the blade is broken or cracked.